Optical system for variable power telescopes



. mam p If 'orrrcansrsrauron'vmmnnarom- ATENT FlcE It Tackaben-y, Rochester. N. Y., to Baruch a Lomb 0ptleal.Company,Boehwater, N. Y.-, a corporation. of New York magnesia, 15,1943, sen-a1 No. 2 7,217

, ,1 em-ment invention relates to variable power I telescopes "and more particlflfl-rly'lelates to provements in the optical systems thereof.

be as near'constant' as possible for all magnificationsof' the inst'rument. when using such a telescope, the observer may adjust the instrument on the gun permanentlyat the mostcomfortable eye distance and upon changing the power of the telescope, no consequential-longi- 1. tudinal readjustmentof the instrument or the 7 posture'of the observer is necessary.' It is also "important that the diameter and the length of ythe telescope-be kept to a minimum so as to reduce the weight and-cost of the instrument. 21- These desirable features should be obtained without sacrificing good optical properties in' the instrument such as chromatic,'comatic, spherical,

and-astigmatic aberrations as well as flatness of *the fleldr I 3 Havingoutlined some of the desirable features of. a variable-power telescope, it is an object of xthis invention to provide :such an instrument .wwhich embodiesall of the above mentioned desirablefeatures. It is also an object to provide 1,, such a. device by the'use of a well corrected'erector lensof relatively short focal length. These and lens l0, an eyepiece lens H for viewing the image formed by the objective lens and an erector lens I! for erecting said image. The erector lens I2 is movable along the optical axis of theinstrument for varying the power or magnification of the telescope between a value of substantially 2.5 and; 4.0, although these values are not exclusive for the purposes of this invention. The erector lensis shown in high power position in full lines and its low power position'is indicated by dotted lines in the drawing. The objective lens [0 comprises the front member, having a positive element I and a negative. element II forming an achromatic doublet, and the rear memberwhichis a fixed collective element III. At the focal plane designing variable power telescopes for the of'sightin'g firearms, it is desirable that I the eye distancei. el,'the' distance between the "telescope eyepiece and the eye of the observer,

a cum; (0188-32) 2 of the objective lens, a diaphragm II is located to-limit the principal rays passing beyond the ob jective lens. 'The eyepiece lens II is composed of two members which are a. doublet having the negative element VII and positive element VIII, and a single collective element 1 In the front focal plane of the eyepiece'lens, a diaphragm I4 is positioned so as to limit the principal rays entering said lens. The erector lens I2 is well corrected'for chromatic, comatic, sphericaL'and astigmatic aberrations and flatness of the field and consists of the two-collective elements IV and VI and an intervening dispersive element V. The eyepiece lens II is also well corrected with regard to the above-mentioned aberrations so that the degree of correction in the erector lens I2 is maintained in the image at the exit pupi1 I5. In the focal plane of the objective lens lo. a reticle I6 is located closely adjacent the-collective element III. An image of reticle I6 is formed by the erector lens H in the-front focal plane of the eyepiece lens Hat M,

In this invention, it has been discovered that by making the focal length of the erector lens I! relatively short. e. g.. substantially 0.25-0.40 times the focal length of the eyenier-e lens I i. the eye distance S9 from the rear surface of element IX to the ex t pu il I5 remains s bstantially constant for all magnifications oi the image. The focal length of the erector lens I2 is also substantially equal to .40 of the focal length of the collective member III. The erector lens I 2 has two conjugate positions, which are the low and high power positions mentioned heretofore, where said lens sharply proiects the image formed by the objective lens at l3 into the front focal plane of the eyepiece lens at it. Due to the short focal length of the erector lens l2, an axial displacement thereof amounting to substantially 0.1-0.2 of the focal length of the eyepiece lens II is suflicient to vary the power of the instrument substantially between 2.5 and 4.0. Due also to said short focal length, the image formed by the erector lens l2 remains substantially focused at H during movement of the lens from one conjugate position to another. The greatest deviation in position of the image formed by the erector lens I! away from the front focal plane at ll of the eyepiece lens "within the above-mentioned range of power is only 0.68 mm. thus necessitating less than a half. diopter' of accommodation at the eye which is easily accommodated. The eye distance Se -a'ctually varies by no more than 2.5 mm. from the nominal value of mm. for all powers of the telescope between 2.5 and 4.0.

Although the aperture stop of most telescopes is the objective lens, the erector lens I! here serves that purpose. By locating the erector lens I! axially close to the place on the optical axis where the cone of image rays approaches minimum diameter between the conjugate image planes at II and I0, the diameter of the erector lens may be reduced to form the aperture stop of the optical system. Since the movement of the erector lens between its conjugate imaging positions is so very short, the aperture si'np remains substantially at the erector lens for v all magnifications of the instrument. These conditions. yielda' relatively large exit pupil with an optimumiy small aperture stop at the erecting lens which contributes greatly to optimum optical performance of the instrument at both extreme and intermediate magnifications. In this way the outer diameter of the telescope is reduced to a minimum, thus providing additional radial space around the erector lens I2 for accommodating the mechanism by which it is moved. In using a variable power telescope embodying this invention for the purpose of sighting a gun, the gunner assumes a steady and comfortable position with his eye at the exit pupil i5 of the telescope. Regardless of the power of the instrument used in sighting the object, the observer's head need not be moved from a fixed position since the exit pupil l5 does not move appreciably for any selected magnification within the range of power of the instrument.

Fig. 2 gives the constructional data for one specific form of the invention as duplicated below in the chart;

11151115111 ND v 115.111 Thlcknesses OBJECTIVE E. F. 118.5 MM.

R1-+ao.7 t|-2.6 1 1.511 54.5

Ri -67.2 is-Li) 11 1.511 35.5

rep-511.11 s1-154.o

lei-+1117 11-5. III 1.517 64.5

Rl- S1=6.6

ERECTOR E. F. 12.6 MM.

R ==m s.=1.4

R|=-7.0 ls=0.4 v 1.049 33.8

Bil-+5.0 S=L0 R.-+1a.5 11-1. 2 v1 1.517 55.0

Ru -fi. 5 S5125. 0 to 19.9

EYEPIECE E. F.31.5MM. v

R1g-'73.3 t1-3.6 VII 1.151 21.7

Riv-+31. 5 i 10. 0 vm 1.011 51.2

Rip-31.5 s.-1.o

Bil-+4 1,-5.5 1! 1.511 51.2

RIP-131.0 si-maomm.

It will be understood from the above description that because of the short focal length of the erector lens l2 and the restriction of its axial movement, the eye distance So is substantially constant while the size oi the instrument is re- 4 ducedto a minimum in accordancewiththeobjects of this invention. Although but one specific format the invention has been shown and described in detail, other forms are possible, and

other values may be substituted in the constructional'data within the range of values specified in the appended claims and changes may be made in the form and arrangement of parts without departing from the spirit of the invention as defined in the appended claims.

I claim:

A variable power telescope comprising fixed objective and eyepiece lenses in optical alignment, a reticle positioned between said lenses in the focal plane of the objective lens, said objective lens including a collective member located adjacent the front side of the reticle, and an erector lens having a focal length which is substantially equal to 0.25-0.40 of the focal length of the eyepiece lens and also substantially equal to .40 of the focal length of the collective member, said erector lens being movable between its two conjugate imaging positions to vary the power of the telescope.

2. A variable power telescope comprising fixed objective and eyepiece lenses in'optical alignment, a reticle located in the plane of the real image formed by the objective lens, said objective lens including a collective member, and an erector lens having a focal length substantially equal to 0.25-0.40 of the focal length of the eyepiece lens and substantially equal to .40 of the focal length of the collective member, said erector lens being movable between its two conjugate imaging positions which are spaced from each other by an amount substantially equal to 0.1-0.2 of the focal length of the eyepiece lens .whereby the image formed by the erector lens in either of said positions remains substantially in the focal plane of the eyepiece lens and the eye distance remains substantially constant for any position of the erector lens.

3. A variable power telescope comprising an objective lens, an eyepiece lens and an erector lens corrected for chromatic, comatic, spherical. and astigmatic aberrations and flatness of field, said lenses having constructional data as defined in the table of values given below, the Roman numerals in the first column designating the component elements in order from front to rear. Nn being the index of refraction of the glass for the D line of the spectrum, V being the dispersion index, R1 to R15 being the radii of the refracting surfaces in order from front to rear and the signs and indicating respectively convex' and concave surfaces relative to incident light, t1-to being the axial thickness of the elements in order from front to rear, 81-80 being axial spaces between the elements in order from front to rear, and E. F. designating the focal lengths of the objective, eyepiece and erector lenses:

Element Nn V Radii Thickness ERECTOR E. F. 12.6 MM.

- Ru 5.15 h- 1.1 IV 1.617 55.0

R1 IN S;- 1.4 R mi 15-- 0.4 v 1.649 33.8

R1e-+13.5 h- 1.2 VI. 1.617 55.0

. 15 EYEPIECE E. F. 37.5 MM.

Ru 73.3 h- 3.5 V11 1.751 37.7

R 31.5 h- 10.0 m. 1.611 57.2

Ru 31.5 Sl- 1.0

Ru -I- 44.0 h- 6.5 It 1.511 57.2

Ru- .137. 0 Sr 100. 0 mm.

ROBERT B. TACKABERRY.

REFERENCES CITED The following references are of record in the me of this patent:

UNITED STATES PATENTS Number Number Altman Nov. 11, 1947 FOREIGN PATENTS Country Date Great Britain Nov. 21, 1928 Great Britain May 23, 1941 

